Radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, satellite radio receivers, and the like. One common type of RF receiver is the so-called superheterodyne receiver. A superheterodyne receiver mixes the desired data-carrying signal with the output of tunable oscillator to produce an output at a fixed intermediate frequency (IF). The fixed IF signal can then be conveniently filtered and converted back down to baseband for further processing. Thus a superheterodyne receiver requires one or more mixing steps.
One well-known problem with the mixing process is that it creates image signals. For some RF systems the level of the image signal is small enough so that designers can rely on the attenuation characteristics of the IF bandpass filter alone to reject image signals. However for other systems the attenuation of the IF bandpass filter is not sufficient. For example satellite radio uses a 2.3 GHz carrier frequency. Each channel has a baseband spectrum from 1 megahertz (MHz) to about 13 MHz, and the adjacent signal spectra can create large image signals. In these systems additional image rejection filtering is required to maintain a sufficient signal-to-noise ratio (SNR) in the desired signal.
One known image signal rejection technique uses a polyphase filter and is performed in two stages. First the input signal is mixed with two separate local oscillator signals that are in quadrature with each other (that is, separated in phase by ninety degrees). Then the two output signals are passed through a phase delay filter to delay the wanted component a certain number of degrees. Then the signals are summed, and the combination of the phase shifts from the mixing process and the phase delay filter causes the desired signal to be passed and its image to be cancelled.
An image rejection mixer can be built to cancel the image frequency. One common approach is to separate two phases of a signal such as the in-phase (I) and quadrature (Q) components, and process these signals using a single phase filter section before recombining the components into an output signal. An alternate approach uses a polyphase filter that processes more than one phase of an input signal. Polyphase filters have been used occasionally in radio applications for many years, and are useful in single sideband (SSB) applications due to their asymmetric frequency characteristics, which can be used for rejection of the unwanted sideband.
In an image-reject mixer, the primary limitations to the potential image rejection are mixer gain mismatching, mixer phase clock accuracy, and polyphase filter rejection. Independent of any mixer gain error and polyphase mixer rejection, a phase clock error of only 0.02 radians (an edge delay of 31 picoseconds at 100 megahertz (MHz) clock rates) limits image rejection to −40 decibels (dB). Thus phase clock accuracy is especially important in providing good image rejection. Moreover several factors present in integrated circuits contribute to phase clock error, including uneven phase clock signal routing lengths, unequal loading on the phase clocks, and non-matching generation logic. New designs that can be implemented in low-cost complementary metal-oxide-semiconductor (CMOS) integrated circuits that have high phase clock accuracy are needed.